The apparatus of this disclosure assists an acoustal well logging apparatus. An acoustic well logging device is normally constructed in a sonde which is lowered into a borehole. Acoustic pulses from a transmitter through well fluid surrounding the sonde and into the formations adjacent to the tool. In idealized fashion, pulses are transmitted parallel to the borehole to be received at spaced acoustic receivers in the sonde. Typically, the sonde is constructed with two receivers, one being closer to the transmitter then the second receiver. For instance, the first receiver might be ten feet from the transmitter. The second receiver might be two feet further or twelve feet from the transmitter. The signal which is transmitted is a fixed frequency tone burst. It is normally an analog signal at the receivers. The amplified output signal recorded against a time base as analyzed for location of the zero crossings of the signal occurring after the transmitted burst is received.
There is a problem in recognizing onset of the received signal. A threshold value must be determined notwithstanding background noise inherent in any analog system and taking into account the very high gain that is normally used in such a system. There is some variation in recognizing the onset of the received signal. For instance, the formations will vary in their ability to transmit signals. There is background or mechanical noise from movement of the sonde. Such noise is inevitably acoustic noise. Also, stray electrical noise from electronics located in the sonde will also impact the recognition of the onset of a signal.
In addition to the recognition of the onset of a received signal, there is also a problem in sorting out the components of the received signal. In ordinary circumstances, a received signal has a first portion which is known as the compression wave or primary wave. That is identified hereinafter as the P wave. Simplistic description of this wave sets forth a model which is primarly wave transmission with particle movement along the axis of propagation of the wave. This direction of movement is perpendicular to the direction of movement of the shear wave which is known hereinafter as the S wave. The shear wave travels slower than P wave. As a matter of general interest, the P wave travels at the rate of about 50 to 100 microseconds per foot of formation in typical formations. By contrast, the shear wave or S wave is slower, typically travelling at a rate of perhaps 80-160 microseconds per foot. The time separation between the P wave and S wave is of some assistance in breaking out the components of the received acoustic wave. There is a third wave which is the fluid or F wave. Typically, the sonde is acoustically coupled to the surrounding borehole by a fluid. There is a fluid wave which travels either in the fluid column or in the fluid which permeates the formation, and its direction of travel is approximately parallel to the borehole. The fluid wave is primarly a compression wave and has a very small component of shear particle motion. Taking into account the three waves just described, ordinarily, a receiver carried on a sonde and spaced from a transmitter on the same sonde will receive the P wave first, the S wave second and the F wave last. While there are great variations in the transmissivity of the three types of waves, it is customary to see them arrive in the sequence of P wave, S wave and F wave last. They will, however, vary markedly in relative amplitude, and will also vary in spacing from one another. For instance, the fluid wave travels typically at a relatively fixed velocity of about 190 microseconds per foot. This velocity is more or less constant. It is a result of transmission of the fluid wave through a fluid which has relatively constant transmission characteristics.
Amplification and subsequent recording of the received signals is important to proper analysis of the data. On the one hand, it is possible to overdrive the amplifier equipment and clip the signal. This results in a loss of signal quality. On the other hand, the signal has a large dynamic range, and it is possible to record the signal with too little gain and thereby lose valuable signal content due to the loss of recorded signal. A happy balance requires operation of the AGC system for the amplifier to provide a suitably amplified signal for recording.
The shape of the signal is important, but the absolute value of the signal is also important. To this end, the gain of the AGC system needs to be represented and recorded along with the signal shape. Further, this must occur dynamically along with the recording of the received signal. In other words, wave shape only will not suffice; peak amplitude is important also.
Data analysis of the recorded acoustic wave form is aided and assisted by recognizing the P, S and F wave onset. It is also helpful to recognize the onset of all three waves separate from one another so that the gain can be adjusted. This particularly prevents overdriving or clipping as will occur when amplifiers are saturated when large signals are observed at the receiver. It is possible that the onset of each of the three wave components is preceeded by a very low null signal wherein the only signal observed is the noise in the system. This null area may separate the three waves, but then again, it may not be present depending on the relative velocity of transmission.
With a view of analyzing the received signal to enable its recordal without clipping or overdriving on the one hand and at sufficient amplitude so as to obtain a fairly large wave form and data, this apparatus is an AGC system for use with an acoustic receiver amplifier in an acoustic logging system. The device is summarized as an AGC control system which observes the onset of the P, S and F wave components (if isolated) and forms signals enabling controlled switching of the attenuation system in the amplifier. So to speak, controlled attenuation is reflected in an AGC signal which can be recorded. The amplifier is switched so that the output signal is confined within certain limits. The output signal is switched from time to time to assure that the entirety of the received signal is recorded at suitable levels.
The apparatus of this disclosure has as one feature a digital gain attenuator. The measure of attenuation is stored so that the gain in the system is recorded as a part of the value of the wave form. Moreover, a pattern of switching is established for the received acoustic signal to enable the P, S and F wave peak values to be observed in the recorded data without overdriving or under amplification.